The tire sidewall is an outer rubber layer that protects that region of the tire from impact and curb scuffing, and provides long-term weathering protection and casing durability. It is formulated to protect the ply, and must possess resistance to weathering, ozone, tearing and both radial and circumferential cracking, while simultaneously providing flex fatigue resistance.
A blend of natural rubber (“NR”) and butadiene rubber has generally been used in the sidewall along with carbon black, a vulcanization system, and a high concentration of antidegradants designed to provide additional resistance to environmental and aging. Natural rubber and polybutadiene, when compounded together, can form two domains. In a tire sidewall compound containing natural rubber and polybutadiene, one rubber will form a continuous phase with dispersed domains of the other rubber serving to arrest any cut growth through the continuous phase. Typically, NR is the continuous phase, although phase inversion can occur. This mixed composition shows better resistance to fatigue and cut growth than if only one rubber is used.
Other problems in tire sidewall formulations arise depending on the specific application. For example, high performance tires for luxury vehicles have the additional requirement of good surface appearance throughout the tire life cycle without loss in durability. Furthermore, commercial truck tires are frequently re-treaded, and the tire sidewall therefore sees extended service life and undergoes excessive heat history, both in service and during the retread vulcanization process. This necessitates good compound aged property retention. A similar point can be made for off-road tires such as those on large dump trucks which have a ton-km-per-hour (TKPH) [0.73 ton-miles-per-hour (TMPH)] rating, where TKPH is the product of the average tire load times the average vehicle speed and the tire TKPH rating is the maximum TKPH at which the tire can be operated to avoid premature tire wear due to excessive heat. Improvements to heat resistance of the rubber compounds used in the sidewall would thus be beneficial for commercial truck tires and off-road tires.
Chemical protectants, such as waxes, antioxidants, and antiozonants, are typically added to the sidewall formulations at optimized levels for effectiveness under both static and dynamic conditions. However, the waxes and antiozonants are continually depleted from the sidewall surface by reaction with ozone, and by physical mechanisms such as curb scuffing and washing. In addition, the chemical antiozonants for rubber are generally costly, staining materials. N,N′-disubstituted-para-phenylenediamines are the most effective antiozonants, particularly the alkyl-, aryl-substituted versions such as N-1,3-dimethylbutyl-N-phenyl-para-phenylene diamine (“6PPD”), for example. Surface discoloration is a particular problem when using 6PPD. It would be desirable to reduce or eliminate expensive antiozonants such as 6PPD while still achieving a black sidewall with good surface appearance and without loss in durability performance over the life of a tire.
Elastomeric compositions having isotactic polypropylene crystallinity, a melting point by DSC of 110° C. or less, a heat of fusion of from 5 to 50 J/g, and comprising at least 60 wt % propylene-derived units, at least 6 wt % ethylene-derived units, and optionally diene-derived units, are described in U.S. Patent Application Publication Nos. (hereinafter “U.S. App.”) 2005/0107529, 2005/0107530, 2005/0107534, 2005/0131142 and International Patent Application Publication Nos. (hereinafter indicated by the “WO” indicator) WO 2005/49670, WO 2005/49671 and WO 2005/49672. Amorphous and partially crystalline (generally referred to as semi-crystalline) polymers can provide elastomeric properties as defined, for example, in ASTM D1566. An important class of elastomers is derived from polyolefins, generally using addition polymerization with a Ziegler-Natta type catalyst system. Some polyolefin elastomers are interpolymers of ethylene, a crystallinity-disrupting α-olefin such as propylene, which provides short chain branches, and optionally small amounts of a polyene, such as a diene, to provide unsaturated short chain branches useful in providing crosslinks between different chains. These interpolymers may be ethylene propylene copolymers (“EP”) not containing units derived from diene, or ethylene propylene diene terpolymers (“EPDM”).
European Patent Nos. (hereinafter indicated by the “EP” indicator) EP 1003814 B1, 946641 A1 and 946640 B1 and U.S. Pat. Nos. (hereinafter “U.S.”) 6,245,856 and 6,525,157, and others disclose polyolefin interpolymers that are elastomers and have crystallinity formed by isotactically-arranged propylene-derived sequences in the polymer chain. This is in contrast with the EP and EPDM interpolymers in current commercial use wherein crystallinity is due to ethylene-derived sequences. The properties of elastomers having crystallinity arising from the microstructure of the propylene units are different in many aspects from known EP and EPDM interpolymer elastomers. Use of dienes for these new propylene-based elastomers has been contemplated. See, for example, WO 2000/69964, including at page 15, lines 18 to 25. Another background reference includes WO 2000/69963.